In many different communications applications, a common signal path is coupled both to the input of a receiver and to the output of a transmitter. For example, in a transceiver, such as a cellular or cordless telephone, an antenna may be coupled to the input of the receiver and to the output of the transmitter. In such an arrangement, a duplexer is used to couple the common signal path to the input of the receiver and to the output of the transmitter. The duplexer provides the necessary coupling while preventing the modulated transmit signal generated by the transmitter from being coupled from the antenna back to the input of the receiver and overloading the receiver.
FIG. 1 shows a conventional front-end circuit 10 such as that used in a cellular telephone, personal communication system (PCS) device or other transmit/receive apparatus. The output of the power amplifier 12 of the transmitter 14 and the input of the low-noise amplifier 16 of the receiver 18 are connected to the duplexer 20, which is a full duplexer. Also connected to the duplexer is the antenna 22.
A typical duplexer, such as duplexer 20, is a three-port device having a transmit port 24, a receive port 26, and an antenna port 28. The antenna port 28 is connected to the transmit port 24 through a first band-pass filter 30 and to the receive port 26 through the series arrangement of a 90° phase shifter 34 and a second band-pass filter 32. The pass bands of the first and second band-pass filters 30 and 32 are respectively centered on the frequency range of the transmit signal generated by the transmitter 14 and that of the receive signals to which the receiver 18 can be tuned. In the example shown in FIG. 1, the band-pass filters are configured such that the high-frequency stop band of the first band-pass filter 30 overlaps the pass-band of the second band-pass filter 32, and the low-frequency stop band of the second band-pass filter 32 overlaps the pass-band of the first band-pass filter 30.
The requirements for the first and second band-pass filters 30 and 32 are quite stringent. The band-pass filters isolate the very weak receive signal generated by the antenna 22 and fed to the input of the low-noise amplifier 16 from the strong transmit signal generated by the power amplifier 12. In a typical embodiment, the sensitivity of the low noise amplifier 16 is about −100 dBm, and the power amplifier 12 can feed power levels of about 28 dBm into the duplexer 20. In this situation, the duplexer 20 attenuates the transmit signal by about 50 dB between the antenna port 28 and the receive port 26 to prevent the residual transmit signal mixed with the receive signal at the receive port 26 from overloading the low-noise amplifier 16.
Duplexer 20 may be used in a personal communication system (PCS) using Code Division Multiple Access (CDMA). CDMA-PCS devices operate in frequency bands at about 1.9 GHz and impose especially stringent requirements on the duplexer performance. The guard band between the portions of the spectrum assigned to the transmit signal and the receive signal is only about 1% of the carrier frequency, i.e. 20 MHz. The bandwidth of the portions of the spectrum assigned to the transmit signal and the receive signal are about 3% of the carrier frequency, i.e. 60 MHz. This means that the band-pass filters are required to have an extremely sharp roll-off. FIG. 2 illustrates the basic arrangement of the transmit and receive bands. The characteristics of the band-pass filters 30 and 32 are identified by reference numbers 36 and 38, respectively, in FIG. 2.
When film bulk acoustic resonators (FBARs) are used in these high quality electronic filters, the performance is sensitive to small amounts of undesired coupling between connections between filter chip and the printed circuit board. These undesired couplings occur on the chip, in the wire-bonds connecting the package to the chip, and in the package itself. In a PCS duplexer, mutual inductances of approximately 1 picoHenry produce noticeable degradation of performance, particularly in the steep transition from pass-band to the reject-band just below pass-band.